Dielectric constant of MgAl_2O_4 spinel and the oxide additivity rule

The dielectric constants and dielectric loss values of five natural and three synthetic spinel single crystals were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: Mg_(1.02)AI_(1.98)Fe_(0.01)O_4 K'=8.355 tan δ=0.0005 Mg_(1.00)AI_(1.98)Fe_0.02)O_4 K'=8.322 tan δ=0.0007 Mg_(1.00)AI_(1.99)Fe_(0.02)O_4 K'=8.203 tan δ=0.0007 Mg_1.01)Al_(1.98)O_4 K'=8.352 tan δ=0.0005 Mg_(0.97)AI_(2.01)Fe_(0.01)O_4 K'=8.302 tan δ=0.0005 Mg_(1.00)AI_(2.00)O_4 K'=8 tan δ=0.0008 Mg_(1.00)AI(2.00)O_4 K'=8.176 tan δ=0.0005 Mg_(0.415)Al_(2.39)O_4 K'8.572 tan δ=0.0011. The agreement between measured dielectric polarizabilities as determined from the Clausius-Mosotti equation and those calculated from the sum of oxide polarizabilities according to α_D(mineral) = Σ_(αD)(oxides) for this group of spinels ranges from + 1.1 to + 3.2%, well above the agreement value of 0.5–1.0% found for well-behaved oxides. Possible reasons for these discrepancies are discussed